Closes https://github.com/flutter/flutter/issues/134969.
Turns out this was literally 1 line. Tested locally:
```shell
dart tools/clang_tidy/bin/main.dart \
--target-variant host_debug_unopt_arm64 \
--lint-all \
--checks="-*,llvm-header-guard"
```
<details>
<summary>Local Test Results</summary>
Here is a tiny snippet showing this is indeed doing something:
```txt
[0:02] Jobs: 0% done, 0/1033 completed, 7 in progress, 1019 pending, 7 failed.
❌ Failures for clang-tidy on /Users/matanl/Developer/engine/src/flutter/display_list/dl_blend_mode.cc:
../../flutter/display_list/dl_blend_mode.h:5:9: error: header guard does not follow preferred style [llvm-header-guard,-warnings-as-errors]
5 | #ifndef FLUTTER_DISPLAY_LIST_DL_BLEND_MODE_H_
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
| USERS_MATANL_DEVELOPER_ENGINE_SRC_OUT_HOST_DEBUG_UNOPT_ARM64_______FLUTTER_DISPLAY_LIST_DL_BLEND_MODE_H
6 | #define FLUTTER_DISPLAY_LIST_DL_BLEND_MODE_H_
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
| USERS_MATANL_DEVELOPER_ENGINE_SRC_OUT_HOST_DEBUG_UNOPT_ARM64_______FLUTTER_DISPLAY_LIST_DL_BLEND_MODE_H
^C
```
</details>
This will likely show failures on CI, so I'm following [our published
directions](https://github.com/flutter/flutter/wiki/Engine-Clang-Tidy-Linter#clang-tidy-fix-on-ci)
to get CI to tell me what needs to get fixed, and won't submit until
there are no warnings at HEAD. As such, not seeking LGTM yet (draft).
---------
Co-authored-by: Zachary Anderson <zanderso@users.noreply.github.com>
20b6399a38